Conductive materials and applied potential to augment methanogenesis for biogas upgradation: Acidogenic synergy, CO2 dynamics and sustainability analysis

Abstract

Methane (CH4) and carbon dioxide (CO2) make up the majority of the raw biogas produced by anaerobic digestion (AD). Increasing the process intensity by concurrently consuming CO2 in the presence of electron donors will enhance the gross calorific value of the biogas. The use of conductive materials (CM) and an electro-methanogenic (EM) process was evaluated to study carbon conversion efficiency to CH4 with simultaneous biogas upgradation. Five reactors (R1-control; R2-Granular Activated Carbon (GAC); R3- Nano Zero Valent Iron (NZVI); R4- Closed Circuit (CC); R5-Applied Potential (AP)) were operated using glucose as a source of carbon (30 g/L) to relatively examine the sustainable effect of CM and EM on methanogenesis. Higher biogas yield with higher CH4 production and accumulation of acetic acid was observed more in the NZVI system followed by AP, CC, GAC, and Control operation. NZVI supplemented significantly added to biogas upgradation (7.6 L) with a concurrent CO2 decrease (87%) as compared to the GAC (3.868 L, 43%) and EM (AP, 4.89 L, 52%; CC, 4.73 L, 49%) but its sacrificial and oxidation nature offsets its translation potential. The electron flux generated in AP and CC reactors had a strong regulatory effect that increased CO2 conversion and utilization towards CH4 compared to other circumstances. The genus-level metagenomic analysis showed a predominance of Clostridium, Ruminococcus, and Acetobacter species in all the reactors except the control. The life cycle assessment (LCA) analysis depicted that the system variation studied showed relatively low CO2 emission in the test condition with NZVI showing the least emission followed by AP and CC. This work offers a method for improving overall CH4 production while overcoming the constraints in methanogenesis, using the influence of in situ carbon flux making the process sustainable.